Following this advice, I've used gnome-disks to create a bootable pen drive containing an image of xenial. After doing so, here's what the pen drive looks like
Presumably the primary partition is
/dev/sdb2, which I obviously don't want to remove. I've also tried resizing /dev/sdb2, but when I attempt to do this, gparted spins forever, issuing the following warning: If I click on ignore, I get an error and the resize fails.
Is there any way to get to use the rest of my drive? My ideal would be to partition the pen drive before making it bootable with gnome-disks, but I'm pretty sure that I can't do that. Thanks for any advice!